Pallets used in industry today are most commonly formed of wood slats and stringers or blocks which are nailed together. While such pallets are functional, they have many disadvantages. First, wood pallets are relatively heavy. Second, wood is not easily cleaned because it absorbs water and other liquids which damage and warp the wood. For these same reasons, the use of wood pallets outside or in wet environments is also limited. Wood pallets are often not consistently dimensioned due to size variations in the wood slats and shrinkage of wood over time, thus preventing such pallets from being stacked or stored in a stable or consistent manner. Also, inconsistent dimensions make automation of these pallets more difficult. Further, the nails used to fasten wood pallets together may cause damage to goods which are stored and transported on the pallet, or wood splinter may result after time. Moreover, once wood pallets are damaged, they have little or no residual value and also necessitate disposal fees.
Replacing wood pallets with plastic pallets has been a goal for many years. The advantages of plastic pallets are many as compared to wood, including greater durability, lighter weight, more consistent dimensions, improved cleanliness, water resistance, higher residual value for recycling, and no nails which may damage products being supported thereon. While many plastic pallets have been attempted, the designs that are able to approximate the strength of wood, to date, have been cost prohibitive and may not have the requisite strength properties. For example, plastic pallets having a solid section, while having favorable stiffness and deflection properties, are heavy and utilize a relatively large quantity of material in their formation. Wood is five to eight times stiffer than a typical plastic used to make pallets. Plastic pallets must either use much more material, be taller, or have reinforcements such as steel rods or glass fillers to compensate for this difference. While conventional plastic pallets have ribbed supports to decrease their weight, they often lack the desired stiffness and low creep properties of the solid pallet, and in order to improve these properties, the height of the pallet must be increased, or reinforcements must be added to the plastic such as steel or other composite reinforcements. Of course, these additions to the plastic cause an increase in material density leading to an even heavier pallet. Further, engineering resins are very expensive resulting in a more expensive pallet.
Another hurdle to overcome with plastic is the cost. Plastic pallets are more expensive than wood by three to five times. This cost can be offset by the number of trips or shipments that can be achieved with plastic versus wood pallets. Another major hurdle is the stiffness of plastic pallets. Racking loaded pallets in warehouses for upwards of 30 days may be common, and the combination of low tensile strength and creep may limit the use of plastic in this manner.
There are three conventional methods of overcoming these weaknesses. The first is to add reinforcement such as steel or a composite to the pallet. This generally adds significant cost and weight and complicates recycling of the pallet. The second is to make the pallet taller. This generally limits the height of a product which is to be stacked on the pallet, as well as the number of pallets that may be stored in a given area. The third is to use reinforced or engineered resins. Again, this adds significant cost and weight. All three obviously limit the acceptance of plastic pallets.
U.S. Pat. No. 3,580,190 provides a partial solution to the stiffness problem by attaching top and bottom sheets 22,24 to the structural network 23, as shown in FIG. 1 thereof. However, this solution does not resolve the bending stiffness problem because large lateral and longitudinal unsupported areas still exist, such as in areas 26, 37, 38, 49 and 50. In other words, this design merely further stiffens the support column areas 67, 68, 69, 97, 98, 99, 28, 30, 32, which already provide substantial stiffness merely as a result of their height. The weakness of this design is apparent in column 6, lines 60-71, where the reference recommends the use of a material having a flexural modulus (or Young's modulus) greater than about 200,000 psi. Such a high modulus material is apparently required because the structure described does not provide significant resistance to deflection along the length and width of the pallet. High modulus materials add substantial cost to the pallet.
Moreover, pallets typically require large openings for receipt of pallet jacks. Because of these large openings, the pallet structure is typically thin and weak and has poor deflection stiffness. Because pallets are exposed to significant abuse, any solution to the stiffness problem must not adversely affect the impact strength of the pallet.
Consequently, an improved pallet is desired which should be reasonably inexpensive, lightweight and sturdy. The improved pallet should also have improved stiffness and creep properties. The pallet should also be able to withstand various environmental conditions to which it may be exposed, particularly moisture. The improved pallet should also be easy to store, have a size compatible with a standard wood pallet, and be reusable. Also, a need exists for improving the stiffness of plastic pallets configured to receive a pallet jack, without reducing impact strength of the pallet. The improved pallet design should also apply to components used in association with pallets.